My son will turn 3 on 3-3-03 so I want to have the #3 as a theme! Any ideas? Was thinking of having 3 of everything...balloons, cakes, scoops of ice cream (small of course!), guests...what else?

Wonder if you can find little matchbox cars with the #3 on them (Dale Earnhart) to use as favors? Of course the price might be preventative unless there are few children attending, like three :)
Maybe you can do a Sesame St.-like game and have the kids identify which picture has three...[whatevers]. They could do it as a group, and you could give prizes for participation. For example, find pictures of animals or other items all on one page, or print them yourself, and say, "which one has THREE?" "The MONKEYS!" they'll say, after counting two polar bear noses and one bird.
You can play the game where they try to drop three clothespins into a bottle, or throw three bean bags into a bucket. If there are a lot of children attending, you could cut cardstock into rectangles, then cut each rectangle (or other shape) into three pieces, give one to each child and they have to find their other two matches. To make it easier each rectangle could have an obviously different picture on it. Or you could number them 1,2,and 3 to help. You could give the team a prize that matches all up first.